Preliminary Investigation on the Effect of Secondary Mirror Assembly on the Optical Performance for the MUltiplexed Survey Telescope

MUltiplexed Survey Telescope (MUST) proposed by Tsinghua University is a 6.5-meter widefield telescope for the ground-based spectroscopic survey. To realize the design target of large field of view of 7 square degrees, the effective aperture of the secondary mirror is preliminarily designed to be 2.4 meters, and the mass of the secondary mirror assembly is supposed to be about 3 tons. Under different zenith angles, the attitude variation of the secondary mirror assembly will affect the stability of the optical axis and result in the spot drift of the focus image on the focal plane. In this paper, a simulation model of the top end assembly is established. Finite element analysis is carried out to investigate the effects of different parameters of the top end assembly on the performance of the optical system, including the vane horizontal offset angle, the vane altitude offset angle, and different forms of ring assembly. Based on the simulation results, an optimized top end assembly is preliminarily obtained and the magnitude of spot drift on the focal plane could be effectively limited at different zenith angles and field of view (FOV).